> @rhattersley You don't need _get_coord_crs() here. And you also use src_coords[0].coord_system later. So it might be simpler and more transparent to copy the pattern used elsewhere.

Myself I'm not sure that "_get_coord_crs(x)" is the same as  "c.coord_system" ?
In my mind, this is about testing the existence of an appropriate *Cartopy* crs, which is not the same as the Iris coord_system and might still not exist even when that does.
I confess though that I'm not certain of the detailed functional differences between the two and what you can in fact rely on here.
( And even if I *am* right about that bit, I now see that means that the error message does not describe the real precise problem)
Can you maybe cast more light on this @rhattersley ?
Thanks